Control of plasmid DNA replication in E. coli will be studied using Pl, a bacteriophage whose prophage is a plasmid, as a model system. Mutants which are maintained at multiple copies per cell will be isolated from Pl carrying multiple antibiotic resistance determinants. These mutants will be analyzed, by DNA-DNA hybridization and by genetic techniques to determine the number and location of the genes involved in the control of plasmid copy number. The mode of replication of the Pl plasmid prophage will be determined by cloning the origin of replication of the Pl prophage, followed by electron microscopy of replicating plasmid DNA molecules to determine the direction of replication. Illegitimate (non-homologus) recombination of the Pl prophage will be studied by analysis of integratively suppressed E. coli dna mutants. The site(s) on Pl at which recombination with the bacterial chromosome occurs will be determined by DNA-DNA hybridization. The effect of the recA allele on illegitimate recombination as well as the possible involvement of insertion sequences IS1, IS2 and IS3 on the recombination events will be determined.